Prospect of solid oxide steam electrolysis for hydrogen production

Transcription

1 Prospect of solid oxide steam electrolysis for hydrogen production Meng Ni a, Michael K.H. Leung b, Dennis Y.C. Leung c a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong nmlemon@hkusua.hku.hk b Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong mkhleung@hku.hk c Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong ycleung@hku.hk ABSTRACT: Hydrogen fuel is an ideal energy carrier to resolve energy crisis and environmental pollution problems. How to produce hydrogen fuel cleanly and economically is of paramount importance toward hydrogen economy. Water electrolysis is a promising technology for large-scale hydrogen production from renewable sources. Compared with alkaline water electrolysis and proton exchange membrane water electrolysis, solid oxide steam electrolysis (SOSE) is advantageous to produce hydrogen efficiently and economically, due to higher electrical efficiency associated with high operating temperature. The fundamentals of SOSE, together with a comparison of SOSE with other high temperature hydrogen production technologies such as direct thermal water splitting, is presented. The above comparison and analysis show the promise of SOSE toward sustainable development. KEYWORDS: Solid oxide steam electrolyzer, Hydrogen production, Thermochemical cycles, Proton exchange membrane, Energy and fuels 1. Introduction Burning of fossil fuels for power generation emits a considerable amount of pollutant and causes serious energy depletion. In order to reduce pollutant emission and ensure energy security, the search for clean, renewable, and reliable alternatives has become increasingly more urgent than ever. Hydrogen has been identified as a promising energy carrier for our sustainable energy supply. When used in a fuel cell, hydrogen can be directly converted into electricity efficiently with water as the only by-product. However, hydrogen is not readily available in nature. Hydrogen must be produced from other hydrogen-containing sources with energy input. Therefore, hydrogen is an energy carrier, not an energy source. Whether hydrogen is renewable or not depends on how it is produced. For hydrogen to be a renewable energy carrier, it should be produced from renewable energy sources [1]. However, currently about 95% of hydrogen is produced from non-renewable sources, i.e. natural gas steam reforming []. Therefore, it is very important to identify and develop feasible methods for practical renewable hydrogen production. Presently, a number of technologies for renewable hydrogen production are under investigation, i.e. water electrolysis, solar thermochemical cycles, photocatalytic water splitting, biomass gasification/pyrolysis, biological conversion etc. [3-5]. Among all these technologies, electrolytic hydrogen production is probably the only practical method for large-scale renewable hydrogen production in the near future. Electrolyzers based on alkaline electrolyte and proton exchange membrane electrolyte has been studied extensively in recent years. Demonstration projects have been carried out worldwide for hydrogen production from solar or wind energy. Due to the great success in solid oxide oxygen ion conductor, solid oxide steam electrolysis (SOSE) becomes an active area for renewable hydrogen production and attracts increasing attention. For this reason, this paper is devoted to evaluate the prospect of solid oxide steam electrolysis for renewable hydrogen production.. Fundamentals of solid oxide steam electrolysis.1. Working principles of SOSE Solid oxide steam electrolysis (SOSE) is a reverse reaction process to solid oxide fuel cell (SOFC). Its fundamental mechanisms are illustrated in Fig. 1. SOSE works at temperature up to 1473K. Steam is fed to the porous cathode. When required electrical potential for water splitting is applied between the two electrodes, water molecules are dissociated to form hydrogen gas and oxygen ions at the cathodeelectrolyte interface, 1/6

2 H O + e H + O (1) - - The hydrogen molecules produced are diffused out of the porous cathode layer to the flow channel, where they are collected. The oxygen ions are transported through the oxygen ion-conducting electrolyte to the anode and subsequently oxidized to form oxygen molecules, - 1 O O + e () Similar to hydrogen, the oxygen molecules produced are diffused from the porous anode to the gas channel for collection. The net reaction can be expressed as 1 HO H + O (3).. Materials for SOSE Figure 1. Working principles of solid oxide steam electrolysis for hydrogen production From the working principles described above, it can be seen that for efficient operation, gas permeation and electron transport through the electrolyte must be inhibited, electrocatalysis should be rapid, the conductivity of each component, i.e. electrolyte, electrode, and interconnect, should be high, and gas transport in the porous electrode should be facile. Therefore, the electrodes must be porous, electronically and ionically conducting, electrochemically active, and have large triple-phase boundaries. Presently, the most widely used cathode and anode materials are porous Ni-YSZ (yttria stabilized zirconia) composite and porous LSM-YSZ composite, respectively. Interconnects are used to link up individual cells in an electrolyzer stack and to keep oxidant and fuel gases separate from one another. The compatitibiliy with electrode and electrolyte in the mechanical, chemical, structural aspects is also required. The electrolyte is a key component of SOSE cell. For stable operation, the solid oxide electrolyte must satisfy the following requirements: (1) gastight to prevent recombination of oxygen and hydrogen, () mechanically stable, (3) high oxygen ion conductivity, and (4) long term stability. Yttria stabilized zirconia (YSZ) is up to now the most widely used oxygen ion conductor for SOSE hydrogen production, as YSZ is stable, gas-tight and has high oxygen ion conductivity. However, the high operating temperature restricts the selection of suitable electrode and interconnect materials, which have to be compatible with the thermal expansion of electrolyte and to withstand highly oxidizing and reducing environment. In order to address this issue, lots of efforts have been made to improve the stability of component materials. Alternatively, there are continuous efforts made to lower the operating temperatures to K, so that the SOSE components, i.e. interconnect, electrode and heat exchangers can be fabricated from relatively low-cost materials. As oxygen ionic conductivity is thermally activated, decreasing operating temperature could significantly increase electrolyte resistance. In order to reduce electrolyte ohmic overpotential, the thickness of electrolyte must be thin while still keep gas-tight. Recent study has shown that even the thickness of electrolyte is only 8 µ m, its ohmic overpotential still remains the most significant voltage loss at an operating temperature of 873K [6,7]. This indicated that for efficient operation, alternative electrolyte materials must be developed. Bismuth oxide (Bi O 3 ) has recently been studied for possible application in SOSE/SOFC (solid oxide fuel cell) [8,9]. It was found that it is much better than YSZ in terms of oxygen ionic conductivity. However, the key problem associated with bismuth is its instability against reduction at low oxygen partial pressure. Scandia stabilized zirconia (Sc O 3 -ZrO, ScSZ) was recently investigated and it was found that its conductivity was twice as much as that of yttria stabilized zirconia (Y O 3 -ZrO, YSZ) electrolyte [10]. However, despite its high oxygen ionic conductivity at lower operating temperature, the oxygen ion /6

3 conductivity of scandia stabilized zirconia decreased with time (aging effect). In recent years, doped lanthanum gallate (LSGM), was also identified as a promising electrolyte material for intermediate solid oxide steam electrolyzer/fuel cells due to its superior oxygen ionic conductivity and negligible electronic conductivity as well as good stability. Due to the great success in material science in recent years, the efficiency and stability of SOSE can be further enhanced in the near future. 3. Comparison with low-temperature electrolysis In addition to SOSE, low-temperature electrolysis is also used for hydrogen production from water. Compared with low-temperature electrolysis, i.e. alkaline water electrolysis and proton exchange membrane water electrolysis, the most significant advantage of SOSE is the reduction of electrical energy demand, as part of energy demand is provided by thermal energy. Figure illustrates the variations of electrolytic hydrogen production energy demand with operating temperature. It can be seen that at 1473K, about 35% energy demand is met by thermal energy. 16 Energy demand (MJ/kg H ) Total, H Electrical, G Thermal, T S Temperature (K) Figure. Energy demand of electrolytic water - splitting for hydrogen production [11] High operating temperature not only results in reduced electrical energy requirement, but also has other advantages. As SOSE cell operates at a very high temperature, the electrochemical reaction kinetics and electronic/ionic transport are very fast; thus, the use of expensive Pt (as catalyst) can be omitted. Furthermore, fast reaction kinetics and carrier transport cause the activation overpotential and ohmic overpotential to be low, leading to low voltage loss. Figure 3 illustrates the J-V characteristics of PEM electrolysis and SOSE hydrogen production under typical operating conditions [1,13]. At J = 6000A/m, a voltage higher than is required for PEM electrolysis. For comparison, the required voltage for SOSE is only about 1.1 V. As the high temperature waste heat from a SOSE cell can be efficiently utilized by a gas turbine, the overall efficiency of SOSE hydrogen production is expected to be higher than those of alkaline electrolysis and PEM electrolysis [15]..5.5 Cell potential, V(V) Model Experiment [14] Electrolyte: Nafion Electrode catalyst: Pt T = 353K, L = 50x10-6 m P = 1 atm Voltage, V(V) Cathode-supported SOSE Typical operating conditions: T = 173 K; P = 1 atm Steam molar fraction: 90% Electrolyte: YSZ Anode: LSM-YSZ Cathode: Ni-YSZ Current density, J (A/m ) Current density, J (A/m ) a. PEM water electrolysis [13] b. Solid oxide steam electrolysis [1] Figure 3 Modelled J-V characteristics of electrolysis for H production under typical operating conditions 3/6

4 In addition to energy demand and efficiency, SOSE has other advantages over low-temperature alkaline and PEM electrolysis. In low-temperature electrolysers, the electrolyte material is either caustic KOH solution or very expensive proton exchange membrane. For comparison, medium cost YSZ is used as electrolyte material in a SOSE cell without any environmental problems. The technological, environmental and economical comparisons of SOSE and low-temperature electrolysis are summarized in Table 1. It can be seen that SOSE hydrogen production has low electrical energy demand, high energy efficiency, and is friendly to the environment. Although its cost is still higher than that of alkaline electrolysis, it is much more economically sound than that of PEM electrolysis. Besides, the cost of SOSE is declining due to continuous R&D efforts in recent years. Compared with low-temperature water electrolysis, SOSE hydrogen production shows great potential. Table 1. Comparison of SOSE with low-temperature water electrolysis (alkaline and PEM) Low temperature water electrolysis Solid oxide steam electrolysis Electrolyte Electrode Alkaline: KOH solution PEM: Proton exchange membrane Alkaline: Ni, Ag PEM: Pt Yttria-stabilized zirconia (YSZ) Ni-YSZ and LSM-YSZ Operating temperature (K) Carriers Alkaline: OH - O - PEM: H + Electrical energy demand 93-95% of the total energy demand About 65% of the total energy demand at temperature 100 o C Energy efficiency Environmental consideration Economic consideration Alkaline: low PEM: higher Alkaline: caustic KOH solution and asbestos PEM: environmentally friendly Alkaline: low cost PEM: very expensive Highest Environmentally-friendly Medium cost 4. Comparison with high-temperature thermochemical hydrogen production As SOSE operates at high temperature, comparison with other high-temperature hydrogen production will be beneficial for justifying its prospect. In recent years, water solar thermolysis and thermochemical water splitting technologies have attracted special attention for solar hydrogen production, as electrical energy consumption is totally avoided [1,16-18]. Water thermolysis is the direct dissociation of water into hydrogen and oxygen gas at a temperature above 000 K. This process is conceptually simple, but operation at such high temperature requires special material selection. When heated by solar concentrators, re-radiation loss is significant due to the very high operating temperature. Besides, hydrogen and oxygen gases are produced simultaneously in the same reactor, leading to safety problems. Gas separation can be achieved by rapid quenching, which causes considerable energy loss. Thermochemical cycles are indirect water-splitting methods for hydrogen production at high temperature. The chemical reactions involved in - step water-splitting thermochemical cycles for hydrogen production can be expressed as y MxOy xm + O (4) 4/6

5 xm + yh O M O + yh (5) x y or x y x' y' M O M O + O (6) M O + H O M O + H (7) x' y ' x y where M is a metal, M x O y and M x ' O y ' are the corresponding metal oxides. Since O and H are produced in two different steps, no gas separation is needed. However, separation of metal produced in the first step is needed to avoid re-oxidation. Fast quenching and membrane separation can be employed. Candidate metal oxides for -step thermochemical cycles include ZnO/Zn, Fe 3 O 4 /FeO, TiO /TiO x, Mn 3 O 4 /MnO, and Co 3 O 4 /CoO [1,19]. Besides -step cycles, 3-step cycles, such as iodine/sulfur (IS) cycles, and 4-step cycles, such as UT-3 cycles, have also received extensive research interests in recent years [0]. The maximum theoretical efficiency of solar thermochemical cycles for hydrogen production is estimated to be about 49.5%, while presently the achievable efficiency is below % [1]. Compared with solar thermolysis, SOSE is safe and efficient. Thermochemical cycle water-splitting is also a promising renewable hydrogen production technology due to its high theoretical efficiency. However, durable materials must be identified for practical hydrogen production. In addition, the multi-step processes add to its complexity and high cost. Obviously, more R&D is required for this technology to be competitive. For comparison, SOSE shows advantages for practical, large-scale renewable hydrogen production. Currently, efficiency of 0-8% for solar-based SOSE hydrogen production is achievable [15]. Compared with thermochemical cycles, SOSE is simple, direct, and economically competitive. More importantly, solid oxide fuel cell (SOFC), the reverse reaction of SOSE, has achieved great success in both material and engineering design aspects. Proper modifications of SOFC could facilitate practical applications of SOSE for hydrogen production. 5. Conclusion Efficient and economical production of hydrogen fuel from renewable resources is a key step toward hydrogen economy. The prospect of SOSE for renewable hydrogen production is evaluated in this paper. The working principles were described. The material issue associated with high operating temperature was discussed. Compared with low-temperature water electrolysis, SOSE is more advantageous due to its high operating temperature. The advantages associated with high operating temperature are: fast electrochemical reaction kinetics, fast electronic and ionic transport rate, and reduced requirement on highgrade electrical energy. As a result, SOSE hydrogen production is more efficient and more economically competitive. Comparison between SOSE and water thermolysis/thermochemical cycles was presented to justify the prospect of SOSE hydrogen production. It was found that SOSE was advantageous over solar water thermolysis. The problems associated with water thermolysis are: explosive mixtures, extremely high operating temperature, and low efficiency. Both SOSE and thermochemical cycles are promising methods for hydrogen production in terms of their theoretical efficiencies. Thermochemical hydrogen production is still at very early development may be a long-term option for solar-hydrogen production. More research is required to make thermochemical cycles feasible for practical applications. For comparison, SOSE is technologically competitive and economically sound. It is expected that SOSE could play an important role in the near term for large-scale renewable hydrogen production. References: [1] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, Potential of renewable hydrogen production for energy supply in Hong Kong, International Journal of Hydrogen Energy, in press, 006. [] A.E. Lutz, R.W. Bradshaw, L. Bromberg, A. Rabinovich, Thermodyanamic analysis of hydrogen production by partial oxidation reforming, International Journal of Hydrogen Energy, 9(8): , 004. [3] M.K.H. Leung, D.Y.C. Leung, K. Sumathy, M.Ni, E. Lau, Feasibility study of renewable hydrogen in Hong Kong, Hong Kong, CLP Research Institute, 004. [4] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water splitting using TiO for hydrogen production, Renewable and Sustainable Energy Reviews, in press, 006. [5] M. Ni, D.Y.C. Leung, M.K.H. Leung, K. Sumathy, An overview of hydrogen production from biomass, Fuel Processing Technology, 87(5): , 006. [6] F. Zhao, A.V. Virkar, Dependence of polarization in anode-supported solid oxide fuel cells on various cell parameters, Journal of Power Sources, 141(1): 79-95, 005. [7] M. Ni, M.K.H. Leung, D.Y.C. Leung, Effect of operational parameters on polarization in anode-supported solid oxide fuel cells, Submitted. 5/6

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